In the oil and gas industry, geophysical prospecting techniques are commonly used to aid in the search for and evaluation of subterranean hydrocarbon deposits. Generally, a seismic energy source is used to generate a seismic signal which propagates into the earth and is at least partially reflected by subsurface seismic reflectors (i.e., interfaces between underground formations having different acoustic impedances). The reflections are recorded by seismic detectors located at or near the surface of the earth, in a body of water, or at known depths in boreholes, and the resulting seismic data may be processed to yield information relating to the location of the subsurface reflectors and the physical properties of the subsurface formations.
Geophysical surveys are used to discover earth structure, mineral deposits, and the subsurface extent of mineral deposits such as oil, natural gas, water, sulphur, etc. Geophysical methods may also be used to monitor changes in the deposit, such as depletion resulting from production of the mineral over the economic lifetime of the deposit. The usefulness of a geophysical study depends on the ability to quantitatively measure and evaluate some geophysical analogue of petrophysical parameters related to the presence of the mineral under consideration.
Changes in the hydrocarbon reservoir over the production life of the reservoir may be detected by changes in a parameter of interest as indicated by temporal changes in seismic body wave characteristics or attributes. The parameter of interest may be any seismic attribute, individually or in combination, of detected seismic body waves which have traversed the reservoir. Seismic attributes are well known in the art. Some examples of attributes are seismic impedance, amplitude, attenuation, frequency, phase, polarity, velocity, dip, azimuth, envelope, etc.
Seismic methods may be applied to production-management monitoring as well as to exploration of hydrocarbon reservoirs. As is well known to geophysicists, an acoustic seismic source at or near the surface of the earth is caused periodically to radiate a seismic wavefield into the earth at each of a plurality of source survey stations. Acoustic seismic sources are usually of the impulsive or swept-frequency type. An impulsive source produces a very sharp minimum-phase wave of very short duration and that somewhat simulates the generation of an impulse. An explosion is an example of such a source.
The swept-frequency or chirp type seismic source may generate a controlled wavetrain to form a relatively long pilot signal such as 2 to 30 seconds to assure sufficient energy is imparted to the earth. The swept-frequency or chirp type source method relies on signal compression to compress the signal and ensure sufficient vertical resolution to resolve the position of subsurface reflectors. Signal compression generally is called deconvolution, with many techniques well known in the art of seismic data processing. Deconvolution of sweep or chirp signals compresses the source signal into a much shorter signal representative of a subsurface reflective boundary. The accuracy and effectiveness of any deconvolution technique is directly related to how well the source signal is known or understood. Most deconvolution operators are derived from statistical estimates of the actual source waveform.
Swept frequency type sources emit energy in the form of a sweep of regularly increasing (upsweep) or decreasing (downsweep) frequency in the seismic frequency range. In addition to upsweeps and downsweeps, various alternative forms of swept frequency signals are well known in the art, for example, so called random sweeps, pseudo-random sweeps or nonlinear sweeps. In a nonlinear sweep, more time may be spent sweeping high frequencies than low frequencies to compensate for high-frequency attenuation in the signal's travel through the earth, or to shape to a desired wavelet. The vibrations are controlled by a control signal, which can control the frequency and phase of the seismic signals.
The acoustic seismic wavefield radiates in all directions to insonify the subsurface earth formations. The radiated wavefield energy is reflected back to be detected by seismic sensors (receivers) located at designated stations also usually located at or near the surface of the earth, but which may also be in the subsurface, for example, in well boreholes (herein, also called wellbores). The seismic sensors convert the mechanical earth motions, due to the reflected wavefield, to electrical signals. The resulting electrical signals are transmitted over a signal-transmission link of any desired type, to instrumentation, usually digital, where the seismic data signals are archivally stored for later processing.
The travel-time lapse between the emission of a wavefield by a source and the reception of the resulting sequence of reflected wavefields by a receiver is a measure of the depths of the respective earth formations from which the wavefield was reflected. The relative amplitudes of the reflected wavefields may be a function (an analogue) of the density and porosity of the respective earth formations from which the wavefields were reflected as well as the formations through which the wavefields propagated. The phase angle and frequency content of returned signals in the reflected wavefields may be influenced by formation fluids, the sought-for minerals or other formation characteristics.
The processed seismic data associated with a single receiver are customarily presented as a one-dimensional time scale recording displaying rock layer reflection amplitudes as a function of two-way wavefield travel time. A plurality of seismic traces from a plurality of receivers sequentially distributed along a line of survey at intervals, such as 25 meters, may be formatted side by side to form a two dimensional (2-D) analog model of a cross section of the earth. Seismic sections from a plurality of intersecting lines of survey distributed over an area of interest provide three-dimensional (3-D) imaging. A series of 3-D surveys of the same region made at successive time intervals, such as every six months, would constitute a 4-D, time-lapse study of the subsurface that would be useful to monitor, for example, the fluid-depletion rate of hydrocarbon reservoir.
From the above considerations, it is reasonable to expect that time-lapse seismic monitoring, that is, the act of monitoring the time-varying characteristics of seismic data associated with a mineral deposit such as a hydrocarbon reservoir of oil or gas over a long period of time, would allow monitoring the depletion of the fluid or mineral content, or the mapping of time-varying attributes such the advance of a thermal front in a steam-flooding operation.
Successful time-lapse monitoring requires that differences among the processed data sets must be attributable to physical changes in the petrophysical characteristics of the deposit. This criterion is severe because changes in the data-acquisition equipment and changes in the processing algorithms, inevitable over many years may introduce differences among the separate, individual data sets from surveys that are due to instrumentation, not the result of dynamic reservoir changes.
In particular, using conventional surface exploration techniques, long-term environmental changes in field conditions such as weather and culture may affect the outcome. If time-lapse tomography or seismic monitoring is to be useful for quantitative hydrocarbon reservoir monitoring, instrumentation and environmental influences that are not due to changes in reservoir characteristics must be transparent to the before and after seismic data sets. Successful time-lapse tomography requires careful preliminary planning.
One way to avoid many time-dependent environmental changes and updated state-of-the-art instrumental changes is to permanently install seismic sources and seismic detectors in one or more boreholes in and around the area of economic interest. Identical processing methods are applied to the data throughout the monitoring period using multiple cross-well (cross-borehole) tomography rather than conventional surface type field seismic operations. One such method is disclosed in U.S. Pat. No. 5,886,255 to Aronstam, filed Oct. 14, 1997 and assigned to the assignee of this invention and which is incorporated herein by reference as a teaching of multiple cross-well tomography.
Another description of wellbores containing permanent downhole formation evaluation systems can be found in U.S. Pat. No. 6,456,566 to Aronstam, filed Jul. 21, 2000 and assigned to the assignee of this invention and all of the contents of which are incorporated herein by reference. The '566 patent teaches the use of minor borehole obstructions as sources of seismic energy.
U.S. Pat. No. 5,406,530, issued Apr. 11, 1995 to Tokuo Yamamoto, teaches a non-destructive method of measuring physical characteristics of sediments to obtain a cross sectional distribution of porosity and permeability values and variations and of shear modulus and shear strength. A pair of boreholes has borehole entries spaced apart from each other at a predetermined distance and a plurality of hydrophones is spaced at predetermined known locations. A pseudo random binary sequence code generator as a source of seismic energy is place in another borehole and activated to transmit pseudo-random wave energy from the source to the hydrophones. Seismic wave characteristics are measured in a multiplicity of paths emanating from the source to the hydrophones using cross-well tomography.
The Yamamoto teaching is primarily directed to use in shallow boreholes for engineering studies. Such holes are less than 100 meters deep, as opposed to oil-field boreholes, which may be two to ten or more kilometers deep. The requirement for an active source to be placed at various levels in the borehole is problematic because the source can damage the hole and interfere with production. Since the seismic equipment must be moved up and down the boreholes, it is impossible to maintain identical recording conditions over an extended time period.
G. W. Winbow in U.S. Pat. No. 4,993,001 issued Feb. 12, 1991, describes a method and apparatus for converting tube waves into downhole body waves for seismic exploration. The equipment comprises a rotary-valve tube wave source for producing swept-frequency tube waves that are injected into tubing or well bore fluid. The tube waves are converted into body waves by an elongate tube wave converter located at a selected position downhole. The tube wave converter comprises an elongate body that substantially fills the well bore or tubing and has a shape that efficiently converts the tube waves to body waves at the selected position downhole. This patent is directed primarily to reverse vertical seismic profiling (RVSP). Winbow acknowledges that it is well known in the art that “nonuniformities in the borehole” cause seismic-wave mode conversions that cause secondary seismic radiation and associated multiples.
Winbow employs a single tube-wave converter to serve as a single source of direct and reflected seismic waves but he must repeatedly reposition the device at spaced-apart intervals down the length of the borehole to get extended vertical coverage as in cross-well tomography. That system thus is difficult to implement for the fixed permanent instrumental installation required for 4-D seismic monitoring operation. A further disadvantage of this scheme is that very high energy tube waves are required, which in turn leads to forming strong background noises as the tube waves enter the borehole and when they impact the end of the borehole.
Another downhole device related to that described in U.S. Pat. No. 4,993,001 is a broadband resonant wave downhole seismic source disclosed by Winbow et al. in U.S. Pat. No. 5,268,537. The device is used to partially or completely block off the borehole and create a fluid-filled borehole cavity. The fluid inside the cavity is oscillated to establish a standing pressure wave which is radiated through the wellbore into the surrounding formation. However, this device functions most effectively at high frequencies (i.e., greater than about 1,500 Hz). It is well known that lower frequencies (less than about 1000 Hz) are preferable for routine reflection seismic and tomographic imaging work.
U.S. Pat. No. 4,671,379, which issued to Kennedy et al. on Jun. 9, 1987 and U.S. Pat. No. 4,834,210, which issued to Kennedy on May 30, 1989, discloses a system in which a pulsed energy source is deployed between two end members. Wellbore fluid occupies the space between the two end members, and the pulsed energy source excites the fluid into oscillation within the borehole space between the two end members. The distance between the two end members is varied as the frequency of the pulsed energy source is varied in order to maintain the system at resonance.
In the Kennedy patents, energy is supplied downhole from the surface through coil tubing connected to a rotary valve located on the device downhole. A column of fluid in the wellbore is excited by the supplied energy to produce a resonant standing wave. This is accomplished by isolating the fluid between two gas bladders to form a column of fluid and exciting the fluid in the column into an oscillating motion with a driver that is in communication with the column of fluid. The fluid is oscillated at the resonant frequency of the column defined by the two gas bladders. During the operation of the invention, it is desirable to sweep the rotary valve through a range of resonant frequencies in order to gain more information about the subsurface of the earth. To perform a frequency sweep and maintain the resonant frequency of the fluid in the column, the length of the column must change for the various frequencies in the sweep. The device accomplishes this task by physically moving the gas bladders during the sweep. In order to maintain the column at ½ wave resonance, the gas bladders are moved during an approximately 45 second sweep through a total distance of about 100 feet. The device must maintain resonance in order to operate effectively.
The source is strong and does not cause borehole damage. The patent states that the device provides a relatively efficient source of energy by operating at the resonant frequency of the column of fluid. However, several problems arise from this approach. First, mechanical movement downhole is necessary because the source must operate at the resonant frequency of the column and cannot vary the resonant frequency without changing the column length. The device must contain relatively complicated downhole equipment in order to vary the length of the column. A system requiring downhole moving parts such as this one is less reliable than desired. Repairs can only be affected by removing the system from the borehole leading to costly down time in operations. A second problem is with the duration of the sweep time of the device. As a sweep is made the length of the column varies. The distance each bladder must travel during a sweep is approximately 50 feet (based on a half wave length of a resonant standing pressure wave). Therefore, the sweep requires an extended time, approximately 45 seconds. It is not possible with this system to produce short sweeps of a few seconds each. Nor is it possible to operate with an impulsive source as the exciter.
The Kennedy et al. patents also describe an alternate embodiment that does not vary the length of the column. In this embodiment, inflatable sleeves surround the conduit between the end elements. These sleeves may be inflated with air causing a change in the apparent compressibility in the borehole fluid. The change in fluid properties changes the resonant frequency of the cavity. However, in this case also, the system can only execute relatively long sweeps and requires downhole moving parts leading to lower field reliability.
There is a need for a system of seismic sources fixed permanently in boreholes that may be used for monitoring time-varying reservoir attributes such as the distribution of the contents of a formation. This system would not interfere with or interrupt production of economic resources. Additionally, there is a need for a system that not only would be used with intentionally generated seismic energy, but could also take advantage of naturally occurring or ambient energy in boreholes, for example fluid flow energy, that may be converted to seismic body waves radiated into earth formations around boreholes.